Thermal transfer ribbon and base film thereof

ABSTRACT

A base film for a thermal transfer ribbon which is a biaxially oriented polyester film comprising polyethylene-2,6-naphthalene dicarboxylate as a main constitutional element, wherein in a temperature-dimensional change curve under load in a longitudinal direction of the film, the dimensional change from the original length of the film at temperatures of up to 200° C. is 1.0% or less and the dimensional change from the original length of the film at temperatures of up to 230° C. is 3.0% or less. This film may have a coating layer of at least one water-soluble or water-dispersible resin selected from the group consisting of an urethane resin, polyester resin, acrylic resin and vinyl resin-modified polyester resin, on one side thereof. The base film gives a thermal transfer ribbon which has excellent adhesion to a sublimation-type ink layer and excellent printing performance without blurred ink at the time of high-speed printing and without wrinkles formed by friction with a head.

TECHNICAL FIELD

The present invention relates to a thermal transfer ribbon and to a basefilm thereof. More specifically, it relates to a thermal transfer ribbonfor use as a transfer material for a thermal transfer printer, which hasexcellent printing performance without blurred ink at the time ofhigh-speed printing and without wrinkles formed by friction with a headand to a base film thereof.

BACKGROUND ART

As a base film for a thermal transfer ribbon for use in a thermaltransfer printer, one having a specific surface roughness (JP-A62-299389) is known.

Of thermal transfer recording materials, demand for a sublimation-typetransfer recording system has been sharply growing because the recordingsystem is capable of outputting a high-quality full-color image withease. The sublimation-type thermal transfer is a system in which only athermally sublimating dye contained in a binder sublimes by heat and isabsorbed into the image receiving layer of paper to which an image istransferred to form a gradation image. Since the temperature of athermal head at the time of printing has become higher along with recentdemand for higher printing speed, the quantity of heat received by athermal transfer printer ribbon has increased. Therefore, thedeformation of a film used as a base film of the ribbon has becomelarger, whereby an unclear printed image is produced or wrinkles areproduced in a ribbon at the time of printing, or in an extreme case,printing is utterly impossible. Therefore, the improvement of printingperformance has been desired.

Further, in sublimation-type thermal transfer, only a thermallysublimating dye contained in a binder sublimes by heat and is absorbedinto the image receiving layer of paper to which an image is transferredto form a gradation image. In order to sublimate only the dye, highadhesion is required between the binder and the base film and, further,the adhesion must not be reduced by environmental changes and thepassage of time. When the adhesion is not sufficient, the binder layertransfers to the paper and greatly impairs gradation, thereby causing an“over-transfer” phenomenon. Since a polyester film generally has highlyoriented crystals, the film has such poor adhesion that an ink layer isnot adhered to the polyester film at all even when it is formed on thefilm directly. Therefore, to improve the adhesion of the polyester filmto the ink layer, a physical or chemical treatment is given to thesurface of the film. However, sufficient adhesion still cannot beobtained even by the treatment.

When the ribbon is separated from an image-received sheet afterprinting, the ink layer may be taken away by the image-received sheetdue to the delamination of the surface of the base film, which may causeabnormal transfer. Therefore, the improvement with regard to this hasbeen desired.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a base film for athermal transfer ribbon which has excellent printing performance withoutblurred ink at the time of high-speed printing and without wrinklesformed by friction with a head.

It is another object of the present invention to provide a base film fora thermal transfer ribbon, which is not heavily deformed at the time ofheating, has excellent adhesion to a thermal transfer ink layer and cangive a transferred image having excellent gradation.

It is still another object of the present invention to provide a thermaltransfer ribbon comprising the above base film of the present inventionas a base film and having the above excellent characteristic properties.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a base film for athermal transfer ribbon, which is a biaxially oriented polyester filmcomprising polyethylene-2,6-naphthalene dicarboxylate as a mainconstitutional element, wherein in a temperature-dimensional changecurve under load in the longitudinal direction of the film, thedimensional change from the original length of the film at temperaturesof up to 200° C. is 1.0% or less and the dimensional change from theoriginal length of the film at temperatures of up to 230° C. is 3.0% orless.

According to the present invention, secondly, the above objects andadvantages of the present invention are attained by a thermal transferribbon comprising the above base film of the present invention and asublimation-type thermal transfer ink layer formed on the base film.

PREFERRED EMBODIMENT OF THE INVENTION

The present invention will be described in detail hereunder.

Polyethylene-2.6-naphthalene Dicarboxylate

The thermal transfer ribbon of the present invention comprisespolyethylene-2,6-naphthalene dicarboxylate as a main constitutionalelement. This polyethylene-2,6-naphthalene dicarboxylate is preferably ahomopolymer whose recurring units are all ethylene-2,6-naphthalenedicarboxylate or a copolymer comprising ethylene-2,6-naphthalenedicarboxylate in an amount of at least 80 mol % of the total of all therecurring units. When the ethylene-2,6-naphthalene dicarboxylate iscontained in an amount of 80 mol % or more of the total of all therecurring units, a film which undergoes only a small dimensional changeat high temperatures can be obtained without impairing thecharacteristic properties of polyethylene-2,6-naphthalene dicarboxylateheavily.

A preferred copolymer component is a compound having two ester-formingfunctional groups in the molecule, as exemplified by dicarboxylic acidssuch as oxalic acid, adipic acid, phthalic acid, sebacic acid,dodecanedicarboxylic acid, succinic acid, isophthalic acid, 5-sodiumsulfoisophthalic acid, terephthalic acid, 2-potassium sulfoterephthalicacid, 2,7-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylicacid, 4,4′-diphenyldicarboxylic acid, phenylindanedicarboxylic acid anddiphenyl ether dicarboxylic acid, and lower alkyl esters thereof;oxycarboxylic acids such as p-oxyethoxybenzoic acid, and lower alkylesters thereof; and glycols such as propylene glycol, 1,2-propanediol,1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol,1,4-cyclohexanedimethanol, p-xylylene glycol, adduct of bisphenol A withethylene oxide, triethylene glycol, polyethylene oxide glycol,polytetramethylene oxide glycol and neopentyl glycol.

The polyethylene-2,6-naphthalene dicarboxylate may have some or all ofits terminal hydroxyl groups and/or carboxyl groups capped with amonofunctional compound such as benzoic acid or methoxypolyalkyleneglycol, or it may be modified by such a trace amount of a polyfunctionalester-forming compound having 3 or more functional groups such asglycerin or pentaerythritol that a substantially linear polymer can beobtained.

Additives

The polyethylene-2,6-naphthalene dicarboxylate base film of the presentinvention may contain such additives as a stabilizer, dye, lubricant,ultraviolet absorber and flame retardant as desired.

To provide preferable slipperiness for the film, it is preferable thatthe film contain a small amount of inert fine particles. Illustrativeexamples of the inert fine particles include inorganic particles such asspherical silica, porous silica, calcium carbonate, silica alumina,alumina, titanium dioxide, kaolin clay, barium sulfate and zeolite; andorganic particles such as silicone resin particles and crosslinkedpolystyrene particles. Synthetic inorganic particles are preferred tonatural ones because they are uniform in size, and inorganic particlesof any crystal form, hardness, specific gravity and color may be used.

The average particle diameter of the above inert fine particles ispreferably in the range of 0.05 to 5.0 μm, more preferably 0.1 to 3.0μm.

The content of the inert fine particles is preferably 0.001 to 1.0 wt %,more preferably 0.03 to 0.5 wt %.

The inert fine particles to be added to the film may consist of a singlecomponent or multiple components having two components or at least threecomponents selected from the above examples.

The time of adding the inert fine particles is not particularly limitedas long as it is before a polyethylene-2,6-naphthalene dicarboxylatefilm is formed. They may be added, for example, during polymerization orbefore film formation.

Thus, a biaxially oriented polyester film having an average surfaceroughness of 0.01 to 0.2 μm can be obtained by adding a lubricant. Whenthe average surface roughness of the film is smaller than 0.01 μm,sufficient slipperiness cannot be obtained, thereby making it difficultto wind the film. When the average surface roughness is larger than 0.2μm and high-speed printing is carried out with a thermal transferprinter, heat conductivity deteriorates and a printed image becomesunclear. When the particle size of the inorganic or organic lubricant tobe added is smaller than 0.05 μm, sufficiently large surface roughnesscannot be obtained, while when it is larger than 5 μm, the film issusceptible to breakage in the stretching step.

Thickness

The thickness of the polyethylene-2,6-naphthalene dicarboxylate basefilm for a thermal transfer ribbon of the present invention ispreferably 0.5 to 10 μm. When the thickness is larger than 10 μm, heatconduction takes time, which is not preferable for high-speed printing.When the thickness is smaller than 0.5 μm, on the other hand, the basefilm has low strength and is inferior in proccessability and a ribbonobtained therefrom is apt to fail to have required strength.

Young's Modulus

The polyethylene-2,6-naphthalene dicarboxylate base film for a thermaltransfer ribbon of the present invention preferably has a total ofYoung's modulus in a longitudinal direction (YMD) and Young's modulus ina transverse direction (YTD) of 1,200 kg/mm² or more, more preferably1,230 kg/mm² or more. When the total is smaller than 1,200 kg/mm², theribbon elongates during running, with the result that a unclear printedimage is apt to be produced or the ribbon is apt to have wrinkles. Theupper limit of the total of the Young's moduli is not particularlyspecified but is preferably 1,600 kg/mm², more preferably 1,500 kg/mm².When the total of the Young's moduli is higher than the above limit, theplane orientation of the molecular chain becomes too high, with theresult of low tear strength, whereby the film is easily broken. Further,this also causes the delamination of the surface of the film.

YMD is preferably 620 kg/mm² or more, more preferably 650 kg/mm² ormore. When YMD is smaller than 620 kg/mm², the orientation of the basefilm becomes low, whereby the base film becomes inferior in heatdimensional stability under load and hardly withstands tension appliedthereto when the base film is used in a ribbon, whereby the ribbon issusceptible to wrinkles or breakage.

The value YMD-YTD is preferably 30 kg/cm or more, more preferably 50kg/mm² or more. Since tension is mainly applied to the longitudinaldirection of the film, orientation in the longitudinal direction ispreferably made higher than that in the transverse direction.

In the present invention, the expression “temperature-dimensional changecurve under load in the longitudinal and transverse directions of thefilm” (will also be referred to as “TMA curve” hereinafter) as usedherein is a curve drawn by plotting the temperatures of the film on theaxis of abscissas and dimensional changes from the original length ofthe film on the axis of ordinates when the film is heated at a fixedtemperature elevation rate while both ends of the film in a longitudinalor transverse direction are held and a fixed load is applied to thefilm.

Temperature Dimensional Change Under Load

In the temperature-dimensional change curve under load in thelongitudinal direction of the biaxially oriented polyester film used inthe present invention, the film has a dimensional change from theoriginal length at temperatures of up to 200° C. of 1.0% or less,preferably 0.6% or less, and a dimensional change from the originallength under load at temperatures of up to 230° C. of 3.0% or less,preferably 1% or less.

When the dimensional change at temperatures of up to 230° C. is morethan 3%, an image is distorted due to the poor dimensional stability ofthe film. Further, when the dimensional change is more than 3% in afilm-shrinking direction, the shrinkage of the film becomes large by theheat of a head at the time of printing and friction between the film andthe printing head becomes large, thereby breaking the film. When thedimensional change is more than 3% in a film-stretching direction, thefilm is wrinkled by the heat of the head at the time of printing,thereby making high-speed printing impossible.

The dimensional change at temperatures of up to 200° C. is 1.0% or less.If it is more than 1.0%, the dimensional stability of the film at thetime of printing with low energy deteriorates, whereby an image isdistorted or printing becomes impossible.

Further, the biaxially oriented polyester film of the present inventionhas a dimensional change from the original length at temperatures of upto 200° C. of preferably 1.0% or less, more preferably 0.6% or less, anda dimensional change from the original length at temperatures of up to230° C. of preferably 3.0% or less, more preferably 1% or less, in thetemperature-dimensional change curve under load in a transversedirection.

Density

The biaxially oriented polyester film used in the present inventionpreferably has a density of 1.3530 g/cm³ to 1.3599 g/cm³, morepreferably 1.3560 g/cm³ to 1.3598 g/cm³. When the density of the film isbelow the above range, a film obtained tends to have low crystallinityand poor heat dimensional stability. When the density is above therange, the crystallinity becomes too high, causing non-uniformity inthickness and deteriorating flatness.

Refractive Index

The biaxially oriented polyester film used in the present inventionpreferably has a refractive index (nZ) in a plane perpendiculardirection of 1.500 or more, more preferably 1.503 or more, much morepreferably 1.505 or more. The upper limit of the refractive index is notspecified but is preferably 1.520 or less. When the refractive index inthe plane perpendicular direction is smaller than 1.500, thedelamination of the surface of the base film easily occurs. When it islarger than 1.520, non-uniformity in thickness becomes large andflatness deteriorates.

Plane Orientation Coefficient

The biaxially oriented polyester film used in the present inventionpreferably has a plane orientation coefficient of 0.010 to 0.040, morepreferably 0.015 to 0.035 measured by an X-ray diffraction symmetricalreflection method. When the plane orientation coefficient is above thisrange, a film which is sufficiently oriented is not obtained easily, andthe film obtained is inferior in heat dimensional stability under loadand cannot withstand tension applied thereto when it is used in aribbon, whereby the base film is susceptible to wrinkles or breakage.When the plane orientation coefficient is below the range, orientationis satisfactory while the delamination of the surface of the film easilyoccurs.

Easily Adhesive Layer

The base film for a thermal transfer ribbon of the present inventionpreferably has a coating layer of at least one water-soluble orwater-dispersible resin selected from the group consisting of anurethane resin, polyester resin, acrylic resin and vinyl resin-modifiedpolyester on the surface of its ink layer side. This coating layer ispreferable because it enhances adhesion between an ink layer comprisinga sublimating dye and a resin binder and a polyester base filmsubstrate. The coating layer may also be formed from an epoxy resin,melamine resin, oxazoline resin, vinyl resin or polyether resin.

The urethane resin comprises as constituent elements a polyol,polyisocyanate, chain extending agent and crosslinking agent asexemplified below. Examples of the polyol include polyethers such aspolyoxyethylene glycol, polyoxypropylene glycol andpolyoxytetramethylene glycol; polyesters such as polyethylene adipate,polyethylene-butylene adipate and polycaprolactone; acrylic polyols, andcastor oil. Examples of the polyisocyanate include tolylenediisocyanate, phenylene diisocyanate, 4,4′-diphenylmethane diisocyanate,hexamethylene diisocyanate, xylylene diisocyanate,4,4′-dicyclohexylmethane diisocyanate and isophorone diisocyanate.Examples of the chain extending agent or crosslinking agent includeethylene glycol, propylene glycol, diethylene glycol,trimethylolpropane, hydrazine, ethylenediamine, diethylenetriamine,4,4′-diaminodiphenylmethane, 4,4′-diaminodicyclohexylmethane and water.

The urethane resin can be produced from the above components by a methodknown per se.

The polyester resin comprises as constituent elements a polycarboxylicacid and a polyhydroxy compound as exemplified below. That is, examplesof the polycarboxylic acid include terephthalic acid, isophthalic acid,orthophthalic acid, phthalic acid, 4,4′-diphenyldicarboxylic acid,2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid,1,4-cyclohexanedicarboxylic acid, 2-potassium sulfoterephthalic acid,5-sodium sulfoisophthalic acid, adipic acid, azelaic acid, sebacic acid,dodecanedicarboxylic acid, glutaric acid, succinic acid, trimelliticacid, trimesic acid, trimellitic anhydride, phthalic anhydride,p-hydroxybenzoic acid, monopotassium trimellitates, and ester-formingderivatives thereof. Examples of the polyhydroxy compound includeethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol,1,3-propanediol, 1,4-butanediol, 1,6-hexanediol,2-methyl-1,5-pentanediol, neopentyl glycol, 1,4-cyclohexanedimethanol,p-xylylene glycol, adduct of bisphenol A with ethylene glycol,diethylene glycol, triethylene glycol, polyethylene glycol,polypropylene glycol, polytetramethylene glycol, polytetramethyleneoxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane,sodium dimethylolethyl sulfonate, potassium dimethylol propionate andthe like. A polyester-based resin can be synthesized through apolycondensation reaction in accordance with a commonly used method byproperly selecting at least one polycarboxylic acid and at least onepolyhydroxy compound from the above compounds. It should be understoodthat the term “polyester-based resin” as used herein comprehends anacryl graft polyester as disclosed by JP-A 1-165633 and a compositepolymer comprising a polyester component such as polyester polyurethaneobtained by extending the chain of a polyester polyol with anisocyanate.

Examples of the acrylic resin include polymers of acrylic monomers,which are enumerated below. The acrylic monomers include alkyl acrylatesand alkyl methacrylates (alkyl group is exemplified by methyl group,ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutylgroup, t-butyl group, 2-ethylhexyl group, cyclohexyl group and thelike); hydroxy-containing monomers such as 2-hydroxyethyl acrylate,2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate and2-hydroxypropyl methacrylate; amide group-containing monomers such asacrylamide, methacrylamide, N-alkylacrylamide, N-alkylmethacrylamide,N,N-dialkylacrylamide, N,N-dialkylmethacrylate (alkyl group isexemplified by methyl group, ethyl group, n-propyl group, isopropylgroup, n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group,cyclohexyl group and the like), N-alkoxyacrylamide,N-alkoxymethacrylamide, N,N-dialkoxyacrylamide andN,N-dialkoxymethacrylamide (alkoxy group is exemplified by methoxygroup, ethoxy group, butoxy group, isobutoxy group and the like),N-methylolacrylamide, N-methylolmethacrylamide, N-phenylacrylamide andN-phenylmethacrylamide; epoxy group-containing monomers such as glycidylacrylate, glycidyl methacrylate and allylglycidyl ethers; acrylic acid,methacrylic acid, acrylonitrile, methacrylonitrile and the like. Theacrylic resin can be produced by (co)polymerizing at least one of theabove monomers in accordance with a method known per se.

The polyester of the vinyl resin-modified polyester resin comprises asconstituent elements a polybasic acid or ester-forming derivativethereof, and a polyol or ester-forming derivative thereof as exemplifiedbelow. Examples of the polybasic acid include terephthalic acid,isophthalic acid, phthalic acid, phthalic anhydride, 5-sodiumsulfoisophthalic acid, 2,6-naphthalenedicarboxylic acid,1,4-cyclohexanedicarboxylic acid, adipic acid, sebacic acid, trimelliticacid, pyromellitic acid, dimer acid and the like. A copolyester resincan be synthesized from two or more of the above acid components.Further, trace amounts of an unsaturated polybasic acid such as maleicacid or itaconic acid and a hydroxycarboxylic acid such asp-hydroxybenzoic acid may be used. Examples of the polyol includeethylene glycol, 1,4-butanediol, diethylene glycol, dipropylene glycol,1,6-hexanediol, 1,4-cyclohexanedimethanol, xylene glycol,dimethylolpropane, poly(ethylene oxide)glycol, poly(tetramethyleneoxide)glycol and the like. Two or more of the above components may beused.

Examples of the vinyl resin used to modify the above polyester includepolymers of vinyl-based monomers, which are enumerated below. Thevinyl-based monomers include monomers containing a carboxyl group orsalt thereof such as itaconic acid, maleic acid, fumaric acid, crotonicacid, styrenesulfonic acid and salts thereof (such as sodium salts,potassium salts, ammonium salts and tertiary amine salts); acidanhydride monomers such as maleic anhydride and itaconic anhydride;vinyl isocyanate, allyl isocyanate, styrene, α-methylstyrene,vinylmethyl ethers, vinylethyl ethers, vinyltrialkoxysilanes,alkylmaleic acid monoesters, alkylfumaric acid monoesters, alkylitaconicacid monoesters, vinylidene chloride, ethylene, propylene, vinylchloride, vinyl acetate, butadiene and the like. The vinyl resin can beproduced by copolymerizing at least one of the above monomers.

The vinyl resin-modified polyester resin can be produced by polymerizinga vinyl-based monomer in a water-soluble or water-dispersible polyesterresin.

A coating solution for forming the above coating layer of awater-soluble or water-dispersible resin may contain an organic solventin such a trace amount that does not affect the water-soluble orwater-dispersible resin and other additives. The coating solution maycontain a surfactant such as an anionic surfactant, cationic surfactantor nonionic surfactant as required. The surfactant is preferably capableof reducing the surface tension of the aqueous coating solution to 40dyne/cm or less and promoting the wetting of a polyester film, asexemplified by polyoxyethylene alkylphenyl ethers, polyoxyethylene-fattyacid esters, sorbitan fatty acid esters, glycerin fatty acid esters,fatty acid metal soap, alkyl sulfates, alkyl sulfonates, alkylsulfosuccinates, quaternary ammonium chloride salts, alkylaminehydrochloric acid, betaine type surfactants and the like.

The coating layer may contain an isocyanate-based compound, epoxy-basedcompound, oxazoline-based compound, aziridine compound, melamine-basedcompound, silane coupling agent, titanium coupling agent,zirco-aluminate-based coupling agent or the like as a crosslinking agentfor improving blocking resistance, water resistance, solvent resistanceand mechanical strength. The coating layer may further contain areaction initiator such as a peroxide or amine, or a sensitizer such asa photosensitive resin if the resin component of an intermediateadhesive layer has a crosslinking reaction point. The coating layer maystill further contain inorganic fine particles such as silica, silicasol, alumina, alumina sol, zirconium sol, kaolin, talc, calciumcarbonate, calcium phosphate, titaniumoxide, barium sulfate, carbonblack, molybdenum sulfide or antimony oxide sol, or organic fineparticles such as polystyrene, polyethylene, polyamide, polyester,polyacrylate, epoxy resin, silicone resin or fluororesin to improveblocking resistance and slipperiness. A dispersant, anti-forming agent,coatability enhancer, thickener, ultraviolet absorber, antistatic agent,organic lubricant, anti-blocking agent, antioxidant, foaming agent, dye,pigment, organic filler, inorganic filler and the like may also becontained as required.

Preferably, this coating solution is applied to one side or both sidesof a polyester film before crystal orientation completes in theproduction process of the polyester film, and the resulting polyesterfilm is dried, stretched and heat set. The coating solution may beapplied separately from the production process of the polyester film.Since dust or the like is easily contained in the coating solution atthe time of coating and a portion containing dust or the like easilycauses a defect at the time of printing, a clean atmosphere is desired,and a preferable film can be produced at a relatively low cost. Fromthese points of view, coating is preferably carried out during theproduction process. The solids content of the coating solution isgenerally 0.1 to 30 wt %, preferably 1 to 10 wt %. The amount of coatingis preferably 0.5 to 50 g per m² of the running film.

Known coating methods can be employed. For example, roll coating,gravure coating, roll brush coating, spray coating, air knife coating,impregnation, curtain coating and the like may be used alone or incombination.

Film Production Process

The polyethylene-2,6-naphthalene dicarboxylate film used in the presentinvention can be produced by biaxially stretching an unstretched filmobtained in accordance with a commonly used method and heat setting it.It can be advantageously produced by carrying out a relaxation treatmentafter heat setting. When the glass transition temperature of thesubstrate polymer of the film is represented by Tg (° C.), theunstretched film is stretched to 2.0 to 6.0 times in longitudinal andtransverse directions at a temperature of Tg to (Tg+60)° C. and heat setat a temperature of (Tg+50) to (Tg+140)° C. for 1 to 100 sec, forexample. Stretching can be carried out in accordance with commonly usedmethods such as an IR heater, rolls or tenter. The film may be stretchedin longitudinal and transverse directions simultaneously orsequentially.

When a relaxation treatment is to be carried out, it is carried outbetween the end of heat setting and the end of winding the film on aroll. Relaxation treatment methods include one in which a 0 to 3%relaxation treatment is carried out in a film width direction byreducing the width of a tenter at the intermediate location of a heatsetting zone, one in which both ends of a film are released and the filmtake-off speed is made slower than the film feed speed at a temperaturehigher than Tg and lower than the fusion temperature of the film, one inwhich a film is heated with an IR heater between two conveyor rollshaving different speeds, one in which a film is carried onto a heatedconveyor roll and the speed of a conveyor roll after the heated conveyorroll is reduced, one in which the take-off speed is made slower than thefeed speed while a film is carried onto a nozzle through which hot airis blown off after heat setting, one in which a film is carried onto aheated conveyor roll after it is taken up by a film-forming machine andthe speed of a conveyor roll is reduced, and one in which the speed of aconveyor roll after a heating zone is made slower than the speed of aroll before the heating zone while it is conveyed through the heatingzone in a heating oven or formed by an IR heater. Any one of the methodsmay be used to carry out a relaxation treatment by making the take-offspeed 0.1 to 3% slower than the feed speed. To make a thermaldimensional change within the range of the present invention, inaddition to the relaxation treatment, a 0 to 3% stretch treatment may becarried out in a film width direction by expanding the width of a tenterin the heat setting zone. This kind of treatment is not limited to theseas long as a thermal dimensional change falls within the range of thepresent invention.

Thermal Transfer Ink Layer

In the present invention, the thermal transfer ink layer is notparticularly limited and known thermal transfer ink layers may be used.That is, the thermal transfer layer comprises a binder component and acoloring component as main ingredients and optionally a softener,plasticizer, dispersant and the like in appropriate amounts.Illustrative examples of the binder component as one of the mainingredients include known waxes such as carnauba wax and paraffin wax,celluloses, polyvinyl alcohols, polyvinyl alcohol partly acetalizedproducts, polyamides, polymer materials having a low melting point andthe like. The coloring agent comprises carbon black as a main ingredientand optionally a dye, or an organic or inorganic pigment. The thermaltransfer ink layer may contain a sublimating dye. Specific examples ofthe sublimating dye include dispersible dyes, basic dyes and the like.

To form the thermal transfer ink layer on the surface of the easilyadhesive layer of a base layer, known methods such as hot melt coating,and solution coating such as gravure coating, reverse coating and slitdie coating in state of a solvent added.

Fusion Preventing Layer

To prevent a thermal head portion from sticking, it is recommended toform a fusion preventing layer of a silicone resin, acrylate having acrosslinkable functional group, methacrylate, polyester copolymerthereof which is crosslinked with an isocyanate, epoxy or melamine,fluororesin, silicone oil or mineral oil on a side devoid of the thermaltransfer ink layer. Further, the fusion preventing layer is preferablyformed before the film is stretched or after the film is stretched in alongitudinal direction. This not only reduces the thermal hysteresis ofthe biaxially oriented polyester film when it is processed into atransfer ribbon but also makes it easy to keep the thermal dimensionalchange properties of the biaxially oriented polyester film within therange of the present invention.

The measurement methods and evaluation methods of property valuesspecified in the present invention are described below.

(1) Thermal Dimensional Change Curve

This is measured using the TMA/SS120C of Seiko Instruments Co., Ltd. Asample having a length of 15 mm and a width of 4 mm is measured using aquartz holder at a measurement temperature of 30 to 280° C. and atemperature elevation rate of 5° C./min under a load of 5 g.

(2) Young's Modulus

A sample having a width of 10 mm and a length of 15 cm is cut out fromthe film and pulled by an Instron type universal tensile tester at achuck interval of 100 mm, a pull rate of 10 mm/min and a chart speed of500 mm/min. The Young's modulus is calculated from the tangent line ofan ascending portion in the obtained load-elongation curve.

(3) Density

This is measured by a float-and-sink method at 25° C. in a densitygradient tube using an calcium nitrate aqueous solution.

(4) Adhesion

The mending tape 810 of Sumitomo 3M Limited is affixed to the surface ofthe ink layer of the manufactured thermal transfer ribbon and strippedoff quickly. The adhesion of the ink layer is evaluated based on thefollowing criteria according to the degree of separation.

5; Ink Layer Does Not Strip Off

4; stripped area of ink layer is less than 10%

3; stripped area of ink layer is 10% or more and less than 30%

2; stripped area of ink layer is 30% or more and less than 80%

1; stripped area of ink layer is 80% or more.

(5) Printability

Printing is carried out on the VY·200 image receiving sheet (trade name,standard paper of Hitachi, Ltd.) with the Hitachi VY·200 printer (tradename, Hitachi, Ltd.) so as to obtain the maximum optical density. Theprintability and wrinkling of the manufactured thermal transfer ribbonare evaluated based on the following criteria.

◯: image is clearly printed

Δ: printing density is not uniform

X: ribbon is wrinkled and printed image is blurred.

(6) Refractive Index

The refractive index is measured using an Abbe's refractometer withsodium D-rays (589 nm) as a light source and calculated from thefollowing expression. nZ is a refractive index in a directionperpendicular to the surface of the film.

(7) Plane Orientation Coefficient

CuKα1 which has been filtered with a nickel filter is measured with theRU200 of Rigaku Denki Co., Ltd. in accordance with a symmetricalreflection method at an output of 40 kV, 50 mA. The strength ratioI(a)/I(b), which is obtained from the base line of a peak (a) appearingat 2θ=21.0 to 24.5° and the base line of a peak (b) appearing at 2θ=24.5to 28° when measured by a symmetrical reflection method using X-raydiffraction, is taken as plane orientation coefficient.

(8) Evaluation of Delamination of Surface of Base Film

Printing is carried out on the VY·200 image receiving sheet (trade name,standard paper of Hitachi, Ltd.) with the Hitachi VY·200 printer (tradename, Hitachi, Ltd.) so as to obtain the maximum optical density. Thedelamination of the surface of the manufactured thermal transfer ribbonis evaluated based on the following criteria.

◯: ink layer itself is not transferred to receiving sheet

X: ink layer itself is transferred to receiving sheet.

EXAMPLES

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

Example 1

Polyethylene-2,6-naphthalene dicarboxylate having an intrinsic viscositymeasured at 25° C. in an o-chlorophenol solution of 0.61 and containing0.4 wt % of spherical silica particles having a particle diameter of 1.2μm was melt-extruded into the form of a film by an extruder and a T dieand forced to make close contact with a water-cooled drum to besolidified by quenching so as to produce an unstretched film. Thisunstretched film was stretched to 4.1 times in a longitudinal direction(mechanical axis direction) at 144° C.

A coating agent having the following composition 1 was applied to theink layer-free side of this stretched film as a fusion preventing layerwith a gravure coater to ensure that the coating film should have athickness of 0.5 μm after dried, and a coating agent having thefollowing composition 2 was applied to the ink layer side of the film asan easily-adhesive layer with a gravure coater to ensure that thecoating film should have a thickness of 0.1 μm after dried. Thereafter,the film was sequentially stretched to 3.7 times in a transversedirection (width direction) at 140° C. and heat set at 240° C. toproduce a biaxially oriented film having a thickness of 5.1 μm (4.5 μmwithout coating layers) without carrying out a relaxation treatment inthe width direction.

(composition 1 of coating agent) acrylic ester 14.0 wt % amino-modifiedsilicone 5.9 wt % isocyanate 0.1 wt % water 80.0 wt % 100.0 wt %(composition 2 of coating agent) acryl-modified polyester 2.78 wt %epoxy resin 0.02 wt % nonionic surfactant 0.20 wt % water 97.00 wt %100.00 wt %

The obtained polyethylene-2,6-naphthalene dicarboxylate base film for athermal transfer ribbon was measured for its Young's moduli inlongitudinal and transverse directions and thermal dimensional changecurves under load in longitudinal and transverse directions to obtainits dimensional change rates at 200° C. and dimensional change rates at230° C.

Thereafter, thermal transfer ink having the following composition wascoated on a side opposite to the fusion preventing layer of the basefilm by a gravure coater to ensure that the coating film should have athickness of 1.0 μm so as to manufacture a thermal transfer ribbon.

(composition of thermal transfer ink) magenta dye (MSRedG) 3.5 wt %polyvinyl acetacetal resin 3.5 wt % methyl ethyl ketone 46.5 wt %toluene 46.5 wt % 100.00 wt %

The printability of the manufactured thermal transfer ribbon wasevaluated. The evaluation results are shown in Table 1.

Example 2

A base film was produced in the same manner as in Example 1 except thatthe stretch ratio in a longitudinal direction was changed to 3.7 timesand one in a transverse direction to 3.9 times.

Thereafter, a thermal transfer ribbon was manufactured by coatingthermal transfer ink in the same manner as in Example 1 and evaluated.The evaluation results are shown in Table 1.

Example 3

A base film was produced in the same manner as in Example 1 except thatthe stretch ratio in a longitudinal direction was changed to 4.8 timesand one in a transverse direction to 3.9 times and that heat setting wascarried out at 245° C. Thereafter, a thermal transfer ribbon wasmanufactured by coating transfer ink in the same manner as in Example 1and evaluated. The evaluation results are shown in Table 1.

Example 4

A base film was produced in the same manner as in Example 1 except thatthe stretch ratio in a longitudinal direction was changed to 5.0 timesand one in a transverse direction to 4.0 times, heat setting was carriedout at 240° C. and the thickness of a film was changed to 3.1 μm (2.5 μmwithout coating layers). Thereafter, a thermal transfer ribbon wasmanufactured by coating transfer ink in the same manner as in Example 1and evaluated. The evaluation results are shown in Table 1.

Comparative Example 1

A base film was produced in the same manner as in Example 1 except thatheat setting was carried out at 210° C. Thereafter, a thermal transferribbon was manufactured by coating transfer ink in the same manner as inExample 1 and evaluated. The evaluation results are shown in Table 1.

Comparative Example 2

A base film was produced in the same manner as in Example 1 except thatthe stretch ratio in a longitudinal direction was changed to 3.0 timesand one in a transverse direction to 3.1 times. Thereafter, a thermaltransfer ribbon was manufactured by coating transfer ink in the samemanner as in Example 1 and evaluated. The evaluation results are shownin Table 1.

Comparative Example 3

A base film was produced in the same manner as in Example 1 except thatthe stretch ratio in a longitudinal direction was changed to 3.6 timesand one in a transverse direction to 3.9 times, heat setting was carriedout at 240° C. and the thickness of a film was changed to 3.1 μm (2.5 μmwithout coating layers). Thereafter, a thermal transfer ribbon wasmanufactured by coating transfer ink in the same manner as in Example 1and evaluated. The evaluation results are shown in Table 1.

Comparative Example 4

Polyethylene terephthalate having an intrinsic viscosity of 0.61measured at 25° C. in an o-chlorophenol solution and containing 0.4 wt %of spherical silica particles having a particle size of 1.2 μm was used.It was stretched in a multiple-stage longitudinal stretching system;that is, it was stretched in a longitudinal direction to 2.2 times at125° C in the first stage, 1.1 times at 125° C. in the second stage and2.3 times at 115° C. in the third stage, which added up to a totalthree-stage longitudinal stretch ratio of 5.6 times, and then stretchedto 3.8 times in a transverse direction in a tenter oven at 110° C.Thereafter, a thermal transfer ribbon was manufactured and-evaluated inthe same manner as in Example 1 except that a fixed-length stretch heattreatment was carried out at 225° C. and then another heat treatment wascarried out while the film was shrunk 6% in a transverse direction at210° C. The evaluation results are shown in Table 1.

Since all the films of Comparative Examples 1 to 4 had poor thermaldimensional stability under load in a longitudinal direction, a ribbonhaving excellent printability could not be obtained from the films.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 C. Ex. 1 C. Ex. 2 C. Ex. 3 C. Ex. 4 mainingredient PEN PEN PEN PEN PEN PEN PEN PET film-forming stretch ratio inlongitudinal number 4.1 3.7 4.8 5 4.1 3 3.6 5.6 conditions direction oftimes stretch ration in transverse number 3.7 3.9 3.9 4 3.7 3.1 3.9 3.8direction of times heat setting temperature ° C. 240 240 245 240 210 240240 225 relaxation treatment % 0 0 0 0 0 0 0 6 thickness of base film μm5.1 5.1 5.1 3.1 5.1 5.1 3.1 5.1 Young's moduli MD kg/mm² 680 670 660 690660 580 610 560 physical TD kg/mm² 580 610 600 600 580 580 610 500properties MD + TD kg/mm² 1260 1280 1260 1290 1240 1160 1220 1060density g/cm³ 1.3580 1.3574 1.3593 1.3582 1.3520 1.3571 1.3573 1.3960dimensional change at 200° C. MD % −0.2 0.0 0.0 −0.4 −0.9 1.0 0.9 −1.5TD % −0.3 0.0 0.0 −0.1 −0.7 0.9 0.8 −0.4 dimensional change at 230° C.MD % 0.2 0.7 0.1 −0.2 −4.0 4.5 3.9 −3.6 TD % −0.2 0.4 −0.1 0.7 −2.5 3.63.2 −0.6 printability ◯ ◯ ◯ ◯ ◯ X X Δ X Ex.: Example. MD: longitudinaldirection of film TD: transverse direction of film C. Ex.: ComparativeExample

Example 5

Polyethylene-2,6-naphthalene dicarboxylate having an intrinsic viscosityof 0.61 measured at 25° C. in an o-chlorophenol solution and containing0.4 wt % of spherical silica particles having a particle diameter of 1.2μm was melt-extruded into the form of a sheet by an extruder and a T dieand forced to make close contact with a water-cooled drum to besolidified by quenching so as to produce an unstretched film. Thisunstretched film was stretched to4.3 times in a longitudinal direction(mechanical axis direction) at 144° C.

The coating agent having the composition 1 used in Example 1 was appliedto an ink layer-free side of this longitudinally stretched film as afusion preventing layer with a gravure coater to ensure that the coatingfilm should have a thickness of 0.5 μm after dried, and the coatingagent having the composition 2 used in Example 1 was applied to the inklayer side of the film as an easily adhesive layer with a gravure coaterto ensure that the coating film should have a thickness of 0.1 μm afterdried. Thereafter, the film was sequentially stretched to 3.5 times in atransverse direction (width direction) at 140° C., heat set at 240° C.and subjected to a 2% relaxation treatment in the width direction toproduce a biaxially oriented film having a thickness of 5.1 μm (4.5 μmwithout coating layers).

The obtained polyethylene-2,6-naphthalene dicarboxylate base film for athermal transfer ribbon was measured for its Young's moduli inlongitudinal and transverse directions, refractive index, planeorientation coefficient, density and thermal dimensional change curvesunder load in longitudinal and transverse directions to obtain itsdimensional change rates at 200° C. and dimensional change rates at 230°C.

Thereafter, transfer ink having the same composition as in Example 1 wascoated on a side opposite to the fusion preventing layer of the basefilm by a gravure coater to ensure that the coating film should have athickness of 1.0 μm so as to manufacture a transfer ribbon.

The printability of the manufactured thermal transfer ribbon wasevaluated. The evaluation results are shown in Table 2.

Example 6

A base film was produced in the same manner as in Example 5 except thatthe stretch ratio in a longitudinal direction was changed to 3.9 timesand one in a transverse direction to 3.9 times and a 1% relaxationtreatment was carried out in a transverse direction.

Thereafter, a thermal transfer ribbon was manufactured by coatingtransfer ink in the same manner as in Example 5 and evaluated. Theevaluation results are shown in Table 2.

Example 7

A base film was produced in the same manner as in Example 5 except thatthe stretch ratio in a longitudinal direction was changed to 4.8 timesand one in a transverse direction to 3.9 times, heat setting was carriedout at 243° C. and a 1% relaxation treatment was carried out in atransverse direction. Thereafter, a thermal transfer ribbon wasmanufactured by coating transfer ink in the same manner as in Example 5and evaluated. The evaluation results are shown in Table 2.

Example 8

A base film was produced in the same manner as in Example 5 except thatthe stretch ratio in a longitudinal direction was changed to 5.0 timesand one in a transverse direction to 4.0 times, heat setting was carriedout at 240° C. and a −1% relaxation treatment (1% stretch treatment) wascarried out in a transverse direction and the thickness of a film waschanged to 3.1 μm (2.5 μm without coating layers). Thereafter, a thermaltransfer ribbon was manufactured by coating transfer ink in the samemanner as in Example 5 and evaluated. The evaluation results are shownin Table 2.

Comparative Example 5

A base film was produced in the same manner as in Example 5 except thatheat setting was carried out at 210° C. Thereafter, a thermal transferribbon was manufactured by coating transfer ink in the same manner as inExample 5 and evaluated. The evaluation results are shown in Table 2.

Comparative Example 6

A base film was produced in the same manner as in Example 5 except thatthe stretch ratio in a longitudinal direction was changed to 3.0 timesand one in a transverse direction to 3.1 times. Thereafter, a thermaltransfer ribbon was manufactured by coating transfer ink in the samemanner as in Example 5 and evaluated. The evaluation results are shownin Table 2.

Comparative Example 7

A base film was produced in the same manner as in Example 5 except thatthe stretch ratio in a longitudinal direction was changed to 3.6 timesand one in a transverse direction to 3.9 times, heat setting was carriedout at 240° C. and the thickness of a film was changed to 3.1 μm (2.5 μmwithout coating layers). Thereafter, a thermal transfer ribbon wasmanufactured by coating transfer ink in the same manner as in Example 5and evaluated. The evaluation results are shown in Table 2.

Comparative Example 8

Polyethylene terephthalate having an intrinsic viscosity of 0.61measured at 25° C. in an o-chlorophenol solution and containing 0.4 wt %of spherical silica particles having a particle size of 1.2 μm was used.It was stretched in a multiple-stage longitudinal stretching system:that is, it was stretched in a longitudinal direction to 2.2 times at125° C. in the first stage, 1.1 times at 125° C. in the second stage and2.3 times at 115° C. in the third stage, which added up to a totalthree-stage longitudinal stretch ratio of 5.6 times, and then stretchedto 3.8 times in a transverse direction in a tenter oven at 110° C.Thereafter, a thermal transfer ribbon was manufactured and evaluated inthe same manner as in Example 5 except that the obtained biaxiallyoriented film was subjected to a fixed-length stretch heat treatment at225° C. and then to another heat treatment while it was shrunk 6% in atransverse direction at 210° C. The evaluation results are shown inTable 2.

TABLE 2 Ex. 5 Ex. 6 Ex. 7 Ex. 8 C. Ex. 5 C. Ex. 6 C. Ex. 7 C. Ex. 8 mainingredient PEN PEN PEN PEN PEN PEN PEN PEN film-forming stretch ratio inlongitudinal number 4.3 3.9 4.8 4.9 4.1 3 3.6 5.6 conditions directionof times stretch ration in transverse number 3.5 3.9 3.9 4 3.7 3.1 3.93.8 direction of times heat setting temperature ° C. 240 240 243 240 210240 240 225 relaxation treatment % 2 1 1 −1 0 0 0 0 physical thicknessof base film μm 5.1 5.1 5.1 3.1 5.1 5.1 3.1 5.1 properties Young'smoduli MD kg/mm² 700 680 650 680 660 580 610 560 TD kg/mm² 550 640 600600 580 580 610 500 MD + TD kg/mm² 1250 1320 1270 1280 1240 1160 12201060 MD − TD kg/mm² 150 40 50 80 80 0 0 60 refractive index nZ 1.5051.507 1.509 1.509 1.493 1.504 1.505 1.495 plane orientation coefficientIa/Ib 0.032 0.020 0.036 0.034 0.009 0.050 0.020 0.050 density g/cm³1.3576 1.3580 1.3593 1.3582 1.3520 1.3571 1.3573 1.3960 dimensionalchange at 200° C. MD % −0.3 0.0 0.0 −0.3 −0.9 1.0 0.9 −1.5 TD % −0.2 0.00.0 −0.1 −0.7 0.9 0.8 −0.4 dimensional change at 230° C. MD % −0.2 0.40.2 −0.1 −4.0 4.5 3.9 −3.6 TD % 0.2 0.7 −0.1 0.8 −2.5 3.6 3.2 −0.6surface ◯ ◯ ◯ ◯ X ◯ ◯ ◯ delamination printability ◯ ◯ ◯ ◯ X X Δ X Ex.:Example MD: longitudinal direction of film TD: transverse direction offilm C. Ex.: Comparative Example

Example 9

Polyethylene-2,6-naphthalene dicarboxylate having an intrinsic viscosityof 0.61 measured at 25° C. in an o-chlorophenol solution and containing0.4 wt % of spherical silica particles having a particle diameter of 1.2μm was melt-extruded into the form of a sheet by an extruder and a T dieand forced to make close contact with a water-cooled drum to besolidified by quenching so as to produce an unstretched film. Thisunstretched film was stretched to 4.1 times in a longitudinal direction(mechanical axis direction) at 144° C.

The coating agent having the composition 1 used in Example 1 was appliedto an ink layer-free side of this longitudinally stretched film as afusion preventing layer with a gravure coater to ensure that the coatingfilm should have a thickness of 0.5 μm after dried, and a coating agenthaving the following composition 2 was applied to the ink layer side ofthe film as an easily adhesive layer with a gravure coater to ensurethat the coating film should have a thickness of 0.1 μm after dried.Thereafter, the film was sequentially stretched to 3.7 times in atransverse direction (width direction) at 140° C. and heat set at 240°C. to produce a biaxially oriented film having a thickness of 5.1 μmwithout carrying out a relaxation treatment in a transverse direction.

Composition 2 of coating agent (acryl+polyester+epoxy)

The composition 2 of the coating agent was as follows. The coating agentconsisted of 42 wt % in terms of solids content of an acrylic resinconsisting 65 mol % of methyl methacrylate/28 mol % of ethyl acrylate/2mol % of 2-hydroxyethyl methacrylate/5 mol % of N-methylolacrylamide; 42wt % in terms of solids content of a polyester resin consisting of 35mol % of terephthalic acid/13 mol % of isophthalic acid/2 mol % of5-sodium sulfoisophthalic acid as acid components and 45 mol % ofethylene glycol/5 mol % of diethylene glycol as glycol components; 6 wt% in terms of solids content ofN,N,N′,N′-tetraglycidyl-m-xylylenediamine as an epoxy-based crosslinkingagent; and 10 wt % in terms of solids content of lauryl polyoxyethyleneas a wetting agent.

The obtained polyethylene-2,6-naphthalene dicarboxylate base film for athermal transfer ribbon was measured for its Young's moduli inlongitudinal and transverse directions and thermal dimensional changecurves under load in longitudinal and transverse directions to obtainthe inclinations of the curves, dimensional change rates at 200° C. anddimensional change rates at 230° C.

Thereafter, thermal transfer ink having the same composition as inExample 1 was applied to a side opposite to the fusion preventing layerso that a coating film should have a thickness of 1.0 μm with a gravurecoater to manufacture a thermal transfer ribbon.

The printability of the manufactured thermal transfer ribbon wasevaluated. The evaluation results are shown in Table 3.

Example 10

A thermal transfer ribbon was produced in the same manner as in Example9 except that a coating agent having the following composition 3 wasapplied to the ink layer side of a film as an easily adhesive layer witha gravure coater to ensure that the coating film should have a thicknessof 0.1 μm after dried.

Composition 3 of coating agent (acryl+polyester+melamine)

The composition 3 of the coating agent was as follows. the coating agentconsisted of 40 wt % in terms of solids content of an acrylic resinconsisting of 75 mol % of methyl methacrylate/22 mol % of ethylacrylate/1 mol % of acrylic acid/2 mol % of N-methylolacrylamide; 40 wt% in terms of solids content of a polyester resin consisting of 30 mol %of terephthalic acid/15 mol % of isophthalic acid/5 mol % of 5-sodiumsulfoisophthalic acid as acid components and 30 mol % of ethyleneglycol/20 mol % of 1,4-butanediol as glycol components; 10 wt % in termsof solids content of methylol melamine, which is a melamine-basedcompound, as a crosslinking agent; and 10 wt % in terms of solidscontent of lauryl polyoxyethylene as a wetting agent.

Thereafter, a thermal transfer ribbon was manufactured in the samemanner as in Example 9 by coating thermal transfer ink and evaluated.The evaluation results are shown in Table 3.

Example 11

A thermal transfer ribbon was produced in the same manner as in Example9 except that a coating agent having the following composition 4 wasapplied to the ink layer side of a film as an easily adhesive layer witha gravure coater to ensure that the coating film should have a thicknessof 0.1 μm after dried.

Composition 4 of Coating Agent (vinyl resin-modified polyester+epoxy)

The composition of the coating agent 4 was as follows. The coating agentconsisted of 84 wt % in terms of solids content of a vinylresin-modified polyester as a main ingredient which consisted of a vinylresin segment comprising methyl methacrylate/isobutylmethacrylate/acrylic acid/methacrylic acid/glycidyl methacrylate and apolyester segment comprising terephthalic acid/isophthalic acid/5-sodiumsulfoisophthalic acid as acid components and ethylene glycol/neopentylglycol as glycol components; 6 wt % in terms of solids content ofN,N,N′,N′,-tetraglycidyl-m-xylylenediamine as an epoxy-basedcrosslinking agent; and 10 wt % in terms of solids content of laurylpolyoxyethylene as a wetting agent.

Thereafter, a thermal transfer ribbon was manufactured in the samemanner as in Example 9 by coating thermal transfer ink and evaluated.The evaluation results are shown in Table 3.

Example 12

A base film was produced in the same manner as in Example 9 except thatthe stretch ratio in a longitudinal direction was changed to 3.7 timesand one in a transverse direction to 3.9 times.

Thereafter, a thermal transfer ribbon was manufactured by coatingthermal transfer ink in the same manner as in Example 9 and evaluated.The evaluation results are shown in Table 3.

Example 13

A base film was produced in the same manner as in Example 9 except thatthe stretch ratio in a longitudinal direction was changed to 4.8 timesand one in a transverse direction to 3.9 times and heat setting wascarried out at 245° C. Thereafter, a thermal transfer ribbon wasmanufactured by coating thermal transfer ink in the same manner as inExample 9 and evaluated. The evaluation results are shown in Table 3.

Example 14

A base film was produced in the same manner as in Example 9 except thatthe stretch ratio in a longitudinal direction was changed to 5.0 timesand one in a transverse direction to 4.0 times, heat setting was carriedout at 240° C. and the thickness of a film was changed to 3.1 μm.Thereafter, a thermal transfer ribbon was manufactured by coatingthermal transfer ink in the same manner as in Example 9 and evaluated.The evaluation results are shown in Table 3.

Comparative Example 9

A base film was produced in the same manner as in Example 9 except thatheat setting was carried out at 210° C. Thereafter, a thermal transferribbon was manufactured by coating thermal transfer ink in the samemanner as in Example 9 and evaluated. The evaluation results are shownin Table 3.

Comparative Example 10

A base film was produced in the same manner as in Example 9 except thatthe stretch ratio in a longitudinal direction was changed to 3.0 timesand one in a transverse direction to 3.1 times. Thereafter, a thermaltransfer ribbon was manufactured by coating thermal transfer ink in thesame manner as in Example 9 and evaluated. The evaluation results areshown in Table 3.

Comparative Example 11

A base film was produced in the same manner as in Example 9 except thatthe stretch ratio in a longitudinal direction was changed to 3.6 timesand one in a transverse direction to 3.9 times, heat setting was carriedout at 240° C. and the thickness of a film was changed to 2.5 μm.Thereafter, a thermal transfer ribbon was manufactured by coatingthermal transfer ink in the same manner as in Example 9 and evaluated.The evaluation results are shown in Table 3.

Comparative Example 12

Polyethylene terephthalate having an intrinsic viscosity of 0.61measured at 25° C. in an o-chlorophenol solution and containing 0.4 wt %of spherical silica particles having a particle diameter of 1.2 μm wasused. It was stretched in a multiple-stage longitudinal stretchingsystem; that is, it was stretched in a longitudinal direction to 2.2times at 125° C. in the first stage, 1.1 times at 125° C. in the secondstage and 2.3 times at 115° C. in the third stage, which added up to atotal three-stage longitudinal stretch ratio of 5.6 times, and thenstretched to 3.8 times in a transverse direction in a tenter oven at110° C. Thereafter, a thermal transfer ribbon was manufactured andevaluated in the same manner as in Example 9 except that the biaxiallyoriented film was subjected to a fixed-length stretch heat treatment at225° C. and then to another heat treatment while it was shrunk 6% in atransverse direction at 210° C. The evaluation results are shown inTable 3.

TABLE 3 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 main ingredient PEN PENPEN PEN PEN PEN film-forming stretch ratio in longitudinal number 4.14.1 4.1 4.1 4.8 5 conditions direction of times stretch ratio intransverse number 3.7 3.7 3.7 3.7 3.9 4 direction of times heat settingtemperature ° C. 240 240 240 240 245 240 relaxation treatment % 0 0 0 00 0 physical thickness of base film μm 5.1 5.1 5.1 5.1 5.1 3.1properties Young's moduli MD kg/mm² 680 680 680 670 660 696 TD kg/mm²580 580 580 610 600 600 MD + TD kg/mm² 1260 1260 1.260 1280 1260 1290density g/cm³ 1.3580 1.3580 1.3580 1.3574 1.3593 1.3582 dimensionalchange at 200° C. MD % −0.2 −0.2 −0.2 0.0 0.0 −0.4 TD % −0.3 −0.3 −0.30.0 0.0 −0.1 dimensional change at 230° C. MD % 0.2 0.2 0.2 0.7 0.1 −0.2TD % −0.2 −0.2 −0.2 0.4 −0.1 0.7 adhesion 5 5 5 5 5 5 printability ◯ ◯ ◯◯ ◯ ◯ C. Ex. 9 C. Ex. 10 C. Ex. 11 C. Ex. 12 main ingredient PEN PEN PENPET film-forming stretch ratio in longitudinal number 4.1 3 3.6 5.6conditions direction of times stretch ratio in transverse number 3.7 3.13.9 3.8 direction of times heat setting temperature ° C. 210 240 240 225relaxation treatment % 0 0 0 6 physical thickness of base film μm 5.15.1 3.1 5.1 properties Young's moduli MD kg/mm² 660 580 610 560 TDkg/mm² 580 580 610 500 MD + TD kg/mm² 1240 1160 1220 1060 density g/cm³1.3520 1.3571 1.3573 1.3960 dimensional change at 200° C. MD % −0.9 1.00.9 −1.5 TD % −0.7 0.9 0.8 −0.4 dimensional change at 230° C. MD % −4.04.5 3.9 −3.6 TD % −2.5 3.6 3.2 −0.6 adhesion 5 5 5 5 printability X X ΔX Ex.: Example MD: longitudinal direction of film TD: transversedirection of film C. Ex.: Example

What is claimed is:
 1. A base film for a thermal transfer ribbon, whichis a biaxially oriented polyester film comprisingpolyethylene-2,6-naphthalene dicarboxylate as a main constitutionalelement, wherein in a temperature-dimensional change curve under load ina longitudinal direction of the film, the dimensional change from theoriginal length of the film at temperatures of up to 200° C. is 1.0% orless and the dimensional change from the original length of the film attemperatures of up to 230° C. is 3.0% or less.
 2. The base film for athermal transfer ribbon according to claim 1, wherein in thetemperature-dimensional change curve under load in a longitudinaldirection of the film, the dimensional change from the original lengthof the film at temperatures of up to 200° C. is 0.6% or less and thedimensional change from the original length of the film at temperaturesof up to 230° C. is 1% or less.
 3. The base film for a thermal transferribbon according to claim 1, wherein in the temperature-dimensionalchange curve under load in a transverse direction of the film, thedimensional change from the original length of the film at temperaturesof up to 200° C. is 1.0% or less and the dimensional change from theoriginal length of the film at temperatures of up to 230° C. is 3.0% orless.
 4. The base film for a thermal transfer ribbon according to claim1, wherein the total of Young's moduli in longitudinal and transversedirections of the film is at least 1,200 kg/mm².
 5. The base film for athermal transfer ribbon according to claim 4, wherein the Young'smodulus in the longitudinal direction of the film is at least 620kg/mm², which is at least 30 kg/mm² larger than the Young's modulus inits transverse direction.
 6. The base film for a thermal transfer ribbonaccording to claim 1, which has a refractive index (nZ) in its thicknessdirection of at least 1.500.
 7. The base film for a thermal transferribbon according to claim 1, which has a plane orientation coefficientof 0.010 to 0.040.
 8. The base film for a thermal transfer ribbonaccording to claim 1, which has a density of 1.3530 g/cm³ to 1.3599g/cm³.
 9. The base film for a thermal transfer ribbon according to claim1, which has a thickness of 0.5 to 10 μm.
 10. The base film for athermal transfer ribbon according to claim 1, which has a coating layerof at least one water-soluble or water-dispersible resin selected fromthe group consisting of an urethane resin, polyester resin, acrylicresin and vinyl resin-modified polyester resin, on one side thereof. 11.The base film for a thermal transfer ribbon according to claim 10,wherein the coating layer is formed by coating a water-soluble orwater-dispersible solution of the above water-soluble orwater-dispersible resin on one side of the film before the completion oforientation and crystallization and drying, stretching and heat settingthe film.
 12. The base film for a thermal transfer ribbon according toclaim 1, which is for a sublimation-type thermal transfer ribbon.
 13. Aprocess for using the base film of claim 1 as a base film for a thermaltransfer ribbon.
 14. A thermal transfer ribbon comprising the base filmof claim 1 and a sublimation-type thermal transfer ink layer on the basefilm.